One of the best app lockers on Android I have come across is "hide it pro"

There are two versions of this app. A free version and a paid pro version which won't give you any ads. Now, why I say it is the best is because of the following reasons

First of all, if u search on Google play store it is available by name "audio manager". This app has a fully functional audio manager interface which effectively helps u manage your all audio needs. Long press on the name " audio manager" will prompt u for a password and viola you are in. Pepper won't even know that you are using a app locker. I think this is the best feature of it.

If u lock an app through this, anyone who wants to access a locked app will be treated by a notification "the app has crashed, do you want to send an error report" if u press "no". You will come out of it, if " yes" then you will have to put the password to access it.

Both pin and text password are supported. Sadly no pattern lock available for this.

Another great feature is "dummy password". Very few people know about this feature. You can set a dummy password so that if someone who knows the app forces you to share your password, you can give him this dummy one. It will unlock bug it won't show anything.

Summing it up, its a awesome tool for hiding sensitive media, apps and secret notes. Do give it a try and you will be amazed.